I'd say the answer is "everywhere". By that I mean many tennis pro shops (those that sell and string rackets) sell nike apparel. However, they usually only sell the Federer and Nadal line of Nike apparel and shoes. They do have some very low-rank wear (i.e. not with so many fancy technologies and stuff). But I'm pretty sure they don't have the polo you want. If you want a Nike polo, it'll be very easy to "go Federer", but not so much with any either polos.